The present invention relates to a microcomputer, a bus control device and a bus control method. More particularly, the present invention relates to a microcomputer having an on-screen display (OSD) circuit and a bus control device and method used when a CPU and the OSD circuit share a same ROM.
A microcomputer having an OSD circuit embedded therein is mounted in AV equipment such as T.V. sets and video decks, to enable on-screen display of various kinds of information including the channel and the volume on a display such as a cathode ray tube (CRT) and a liquid crystal display (LCD).
In some of OSD circuit-embedded microcomputers, the OSD circuit and a CPU share a same ROM, as disclosed in Japanese Laid-Open Patent Publication Nos. 11-102352 and 11-134291, for example. In such microcomputers, bus control is performed to permit alternate access of the CPU and the OSD circuit to the ROM at the proportion of 1:1, in which read of instruction data by the CPU and read of display data by the OSD circuit are performed in a time-sharing manner.
In the microcomputers described above, the alternate access of the OSD circuit and the CPU to the ROM at the proportion of 1:1 must be followed even when the need arises for the OSD circuit to access the ROM. Therefore, if the number of characters and the number of colors of a character to be displayed increase, and thus a large amount of display data must be read from the ROM to the OSD circuit, the OSD circuit may fail to access the ROM sufficiently.